Presentation on theme: "Supernova. Explosions Stars may explode cataclysmically. –Large energy release (10 3 – 10 6 L ) –Short time period (few days) These explosions used."— Presentation transcript:
Explosions Stars may explode cataclysmically. –Large energy release (10 3 – 10 6 L ) –Short time period (few days) These explosions used to be classified as novas or supernovas. –Based on absolute magnitude They are now all called supernovas.
Hydrogen Lines Supernovas are classified by their emission spectra. –Historical classification –Not related to mechanism The initial classification is based on hydrogen. Secondary classification is based on other elements. –Silicon absorption –Helium emission
Mass Relations Stars on the HR diagram line up according to mass. The time on the main sequence is spent burning hydrogen. –Massive stars burn faster 10 M 3 M 0.02 M 0.5 M
Giants When core hydrogen is exhausted helium burning begins. –Degenerate gas core 10 8 K Helium fusion through triple alpha causes a helium flash. –Rapid expansion 100 x R Aldebaran Capella giants
Degenerate electrons The nuclei from fusion are separated from their electrons. –Filled fermi states with degenerate electrons –Provides opposing force to gravity The energy of contraction blows off outer layers of star. inward force of gravity outward force of electrons
Dwarves Giants that exhaust their core helium become white dwarves. –Planetary nebulas Isolated white dwarves slowly cool due to lack of further fusion. white dwarves giants
Binary Dwarves White dwarves can occur in binary stars. –One star ages faster –Original detection White dwarves continue gravitational pull on companion. –Tidal forces Sirius image from Chandra - NASA
Binary Explosions A binary can transfer gas from a giant to a white dwarf. If the white dwarf exceeds M CH, gravity will exceed electron repulsion. It will explode into a type I supernova. –Star-sized fusion bomb giant star gas pulled to partner white dwarf supernova
Binary Life Cycle Close binary stars will evolve at different times. The massive star will form a white dwarf first. The second star goes giant and engulfs white dwarf. –Material from the second star is also blown away supernova 1-3 M 4-9 M 1-3 M 1.5 M
Core Fusion For high mass stars fusion continues beyond helium fusion. Each fusion stage requires higher temperatures and pressures and takes place in deeper layers. Fusion steps –Hydrogen to helium –Helium to carbon –Carbon to oxygen –Oxygen to neon –Neon to silicon –Silicon to iron
Supergiants Massive stars can sustain helium burning and that are brighter than expected are large and are called supergiants. –M > 5-8 M Rigel Betelgeuse supergiants
Gravitational Binding The change in gravitational energy is released during collapse. –From 1 M , r = 1000 km –To r = 10 km The estimate is an order of magnitude greater than the amount needed for nuclear changes. –90% available for release
Total Energy The energy released by the collapse of a core is great. –Optical: 10 42 J in weeks –About 10 10 times the Sun –Equal to some galaxies
Death of Supergiants A supergiant with more than 8 M will oscillate in temperature becoming more luminous. Eventually the core is so collapsed by gravity that the electrons cannot hold the core apart. A star like this will become a type II supernova. Sun supernovae
Neutrino Production The core can cool by producing neutrinos. –Plasma at 10 11 K –Opaque to photons Neutrinos can carry kinetic energy. –Hot enough for all three types –Pair production dominates
Stellar Explosion When gravitational force exceeds the electron repulsion, the core collapses immediately. The energy is released as photons and mostly neutrinos. The outward energy hits collapsing material and the star explodes.
Supernova Remnants The supernova core collapse is at 200 billion K. The photons are energetic enough to break up iron nuclei. The particles from the broken nuclei fuse with iron to create heavy elements. This matter goes to form new stars and planets.
Nuclear Force Neutron stars forms when the core mass exceeds the Chandrasekar mass: 1.5 M . –Photodisintegration: 1.4 x 10 45 J –Electron capture: 1.6 x 10 45 J Nuclear forces stop further collapse. –Reach nuclear density r 0 = 1.2 x 10 -15 m nuc = 2.3 x 10 17 kg/m 3
Pulsars Neutron stars create very large magnetic fields. –Spin faster with collapse –Up to 30 Hz They can be observed as repeating flashes of light as the magnetic poles point towards us.
Rotation Time Minimum period is found by balancing gravity and centripetal force. –Fast rotation from high density The period decreases with time. –Magnetic dipole radiation –Predict 1200 years for Crab pulsar
X-rays The surface gravity creates tremendous accelerations. –Accelerating electrons radiate photons –Radiate as x-rays X-ray telescopes in orbit can spot neutron stars in supernova remnants.
X-ray Pulsars Pulsars also emit x-rays. –Blink at characteristic period –Crab nebula period 33 ms Crab nebula offCrab nebula on